Environmental and energy concerns have stimulated the development of alternative fuels for use in automobiles. For example, alcohol fuels such as ethanol and methanol may be used. Flexible-fueled vehicles are capable of operating on gasoline, alcohol fuel, or any combination of the two fuels. Modifications to the engine are necessary when operating on different fuels because each fuel has different characteristics. An engine operating on ethanol or E85 fuel requires approximately 1.4 times the amount of fuel relative to gasoline due to a lower energy content of ethanol. E85 refers to a fuel blend of 85% ethanol and 15% gasoline. Similarly, E0 refers to a fuel with a 100% gasoline composition.
Air/fuel ratio in internal combustion engines is considered to be a ratio of mass air flow rate to mass fuel flow rate. The ratio corresponding to complete oxidization of the air/fuel mixture is referred to as stoichiometric. If the air/fuel ratio is less than stoichiometric, the engine is said to be operating rich. In other words, too much fuel is being delivered relative to the amount of air. Conversely, if the air/fuel ratio is more than stoichiometric, the engine is said to be operating lean. A lean condition indicates that not enough fuel is being delivered relative to the amount of air. An engine operating on an alcohol fuel requires a lower air/fuel ratio than an engine operating on gasoline. As the percentage of alcohol in the fuel increases, more fuel is required to lower the air/fuel ratio.
An oxygen sensor may be used to determine the percent alcohol content of the fuel in the engine. The oxygen sensor relays fuel composition information to the engine controller so variables such as air/fuel ratio can be adjusted accordingly. However, oxygen sensors take a predetermined amount of time to warm up, particularly in cold start conditions. Fuel composition learning systems cannot be used until the oxygen sensor is functioning correctly. Therefore, a potential for drivability deficiencies exists during a warm-up period if the vehicle has recently been filled with a fuel blend that differs from the previous fuel blend in the fuel tank.